This relates to sensors and, more particularly, to ambient light sensors for electronic devices.
Cellular telephones and other portable devices with displays such a tablet computers sometimes contain ambient light sensors. An ambient light sensor can detect when a portable device is in a bright light environment. For example, an ambient light sensor can detect when a portable device is exposed to direct sunlight. When bright light is detected, the portable device can automatically increase the brightness level of the display to ensure that images on the display remain visible and are not obscured by the presence of the bright light. In dark surroundings, the display brightness level can be reduced to save power and provide a comfortable reading environment.
The fundamental limitation to ambient light sensor sensitivity is photo sensor leakage current (or “dark” current). With conventional devices, ambient light sensors can be implemented using first and second silicon photosensors (i.e., two photodiodes). The first photodiode is exposed to ambient light, whereas the second photodiode is a metal-covered photodiode that does not receive any ambient light. The first photodiode is used to measure a total current while the second photodiode is used to measure a leakage current. The leakage current is subtracted from the total current to compute a final leakage-current-compensated output value.
Computing leakage-current-compensated light levels in this way, however, is costly. In this conventional approach, the first and second photodiodes are of the same size (i.e., each of the first and second photodiodes take up the same amount of area on an integrated circuit substrate). A single photodiode can be at least 100,000 times larger than a single transistor (as an example). The use of two photosensors of the same size therefore takes up a significant amount of die area.
It would therefore be desirable to be able to provide ambient light sensors with reduced area requirements for electronic devices.